Provide fill_value, dtype to fetch_raster_window

[TomAugspurger]

Closes #113

[TomAugspurger]

Using the keyword window_dtype, instead of dtype, to avoid a conflict with blockwise using dtype as well.

[gjoseph92]

Thanks for the issue and the PR. Oddly I also happened to notice this a few days ago and fixed it on a local branch I was working on, but hadn't gotten around to pushing it up.

The one issue with this approach is that fill_value can currently be passed in as None (a never-used feature that results in whatever nodata value is written into the GeoTIFF's metadata as the nodata value acts as the fill value). When that's the case, np.array(fill_value, window_dtype) will fail (unless window_dtype is float, in which case you'll get NaN).

On my branch I just removed this feature. Why don't you take a look at #115 and see if that'll work for you?

[TomAugspurger]

Yep, that seems to work.

I think losing fill_value=None is OK, since that will ideally be included in the STAC metadata.